The present invention relates to a device and method for the detection and control of the light intensity of a laser light source used in a laser printer or the like.
In a conventional laser printer, output from a laser light source is guided via an optical system to a rotating or oscillating mirror, and is scanned across a photoconductive drum by the mirror. The photoconductive drum is thereby exposed, and an electrostatic latent image (to be transferred to a recording sheet) is formed on the photoconductive drum.
Since the density of the electrostatic latent image changes with the light intensity of the laser flux output from the laser light source, the power of the laser flux must be controlled to a predetermined magnitude. Conventionally, in laser printer systems, the intensity is controlled according to an intensity detector provided as part of the laser light source or at the laser light source.
However, the intensity of the laser flux guided to the photoconductive drum changes as it passes through the optical system between the laser light source and the photoconductive drum, or is changed during transmission due to environmental variations, causing the printing precision of the laser printer to become unstable.
One method of controlling the intensity includes splitting the light flux emitted from light source into a monitor light flux and a main light flux. The light intensity of the main light flux is controlled by detecting the light intensity of the monitor light flux. This kind of light intensity controlling device is widely used in the field of optical disk devices. A light flux emitted from a light source (such as a semiconductor laser) is split by a light splitting element (such as a beam splitter), and one of the split light fluxes is used as a monitor light flux. The other (main) flux is introduced to an optical disk, and is modulated to write information on the disk. This method is known as automatic power control (APC).
However, in the conventional light intensity controlling device, where more than one light source emit light fluxes having differing states of polarization, the intensity on an image plane can not be kept at an predetermined value by the APC operation. This is also the case in a device using one or more light sources, where the state of polarization of the light flux from the light source(s) is changed due to environmental fluctuations.
In general, the transmissivity or reflection index of an optical splitting element, and the indices of other optical elements guiding the main light flux onto the image plane, have polarization dependency. In other words, depending on the state(s) of polarization of an incident light flux, the light energy (light quantity) of the transmitted light or reflected light varies. In particular, since only an extremely small portion of the light quantity is split to be used as a monitor light flux in comparison to the main light flux (in order to secure the optical utilization efficiency on the main light flux side) the light intensity of the monitor light flux may change significantly according to the state(s) of polarization of incident light. Furthermore, the balance between the light intensity of the main light flux and that of the monitor light flux, when split by an optical splitting element, changes according to the state(s) of polarization of incident light flux.
Therefore, when the balance between split light fluxes is lost, a conventional APC operation based on the light intensity of the monitor light flux fails to keep the light intensity of the main light flux on the image plane at a predetermined level.
This problem is particularly significant in an optical system utilizing optical fibers. In optical transmission, general-use optical fibers do not maintain a plane of polarization (although there is a case where the polarization direction of incident linearly polarized light is orthogonal to that of outgoing linearly polarized light). The change in the plane of polarization are caused by the bending and twisting of the optical fibers (as is necessary for installation).
For example, a situation where the problem of a loss of a plane of polarization is significant is in a multi-beam optical system in which light fluxes emitted from several light sources are introduced by optical fibers corresponding to each light source, and spot light sources are formed for each light source, and spot light sources are formed for each light source. If the light fluxes from the optical fibers are split by a beam splitter to obtain a monitor light flux, even when the state of polarization of light entering the respective optical fibers from each light source may be uniform, the state of polarization of each light flux emitted from the respective optical fibers (and incident into the beam splitter) differs. Therefore, even though an APC operation may be performed on the basis of the light intensity detected from the monitor light flux, it is difficult to keep the light intensity of the main light flux on an image plane constant.